What is Pyuria?
Pyuria is the presence of white blood cells in the urine, typically defined as ≥10 white blood cells per mm³ in an enhanced urinalysis or ≥5 white blood cells per high-power field on a centrifuged urine specimen. 1
Definition and Quantification
The standard threshold for pyuria is ≥10 WBCs per mm³ in uncentrifuged urine using hemocytometer counts, or ≥5 WBCs per high-power field when examining centrifuged urine sediment under microscopy 1
Some evidence suggests that higher cutoffs may be more specific—a threshold of >25 WBCs per high-power field provides the optimal balance of sensitivity and specificity for detecting bacteriuria, though this still yields inadequate diagnostic accuracy when used alone 2
Detection Methods
Pyuria can be identified through multiple approaches 1:
- Leukocyte esterase dipstick testing (moderate sensitivity of 83%, specificity of 78%)
- Microscopic examination of urine sediment
- Hemocytometer cell counts of uncentrifuged urine
- Automated urinalysis using flow imaging analysis technology
Clinical Significance and Interpretation
Pyuria represents the host's inflammatory response within the urinary tract and is a hallmark finding that helps distinguish true urinary tract infection from asymptomatic bacteriuria. 1
Key Diagnostic Principles:
The absence of pyuria has excellent negative predictive value approaching 100% for ruling out true UTI in most clinical scenarios 1
Pyuria alone does not confirm infection—only 25-54% of patients with pyuria actually have bacteriuria, with rates increasing as WBC counts rise 2
When combined with nitrite testing, diagnostic sensitivity increases to 93% 1
Important Clinical Context:
In catheterized patients, pyuria is far less reliable for diagnosing catheter-associated UTI, with sensitivity of only 37% when using the standard >10 WBCs/µL cutoff 3
Pyuria without bacteriuria may indicate non-infectious inflammation, tuberculosis, chronic kidney disease, or other non-infectious conditions 1
In CKD patients, asymptomatic pyuria occurs in 30.5% of cases (51.4% in hemodialysis patients), with over 70% being sterile 4
In pediatric patients, up to 20% of febrile infants with culture-proven pyelonephritis may not have pyuria on initial urinalysis 1
Common Pitfalls to Avoid
Never treat bacteriuria without pyuria or symptoms—this represents asymptomatic bacteriuria and leads to unnecessary antibiotic use, increased resistance, and potential adverse effects 1
Do not use pyuria as the sole criterion for obtaining urine cultures in catheterized patients, as most catheter-associated infections are asymptomatic and pyuria correlates poorly except with gram-negative infections 3
Always correlate pyuria with clinical symptoms such as dysuria, frequency, urgency, fever, gross hematuria, or new/worsening urinary incontinence before diagnosing true UTI 1
Recognize that sterile pyuria exists—the majority of urinary WBCs in CKD patients are neutrophils even without infection, though the percentage is lower in sterile pyuria 4